Removing lid of box, aide finds chemical compound (5)

I believe the answer is:

oxide

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'chemical compound' is the definition.
(I know that oxide is a type of chemical compound)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for oxide that I've seen before include "Rust, for instance" , "Compound of two elements, one with atomic number 8" , "Oxygenous compound" , "Compound of oxygen with another element" , "Rust is iron -" .)

I've seen this clue in The Irish Times.

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